In recent years, blogs, which is short for a “Web Log”, have been widely used in the Internet. Blogs are updated periodically by users and have a function for accepting comments from third parities and a function for accepting trackback from other blogs. In blogs, people have a lively exchange of views using comments from third parties and trackbacks between blogs. In a chat, users who log into a chat room may have a lively exchange of views in real time. Examples of these techniques are described in Japanese Patent Application Laid-Open Nos. 2006-244494 and 2007-140749